Overhaul migration prompts with JAX best practices and MaxText support

Rewrites and expands the migration prompt library with:

- Comprehensive JAX/Flax best practices (KV cache, causal conv1d,
  static shapes, attention masking, rotary embeddings, etc.)
- Single-file and multi-file conversion prompts
- MaxText-specific conversion prompts (YAML overlay, custom layers)
- Hallucination prevention rules
- Flax scoping and naming conventions

Also updates models.py with additional model configuration support.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: gvanica